This invention relates to ultrasonic imaging systems, and more particularly to a simplified sector scanner with a fixed focus transducer array assembly in combination with a dynamic aperture control for acceptable resolution at both shorter and longer ranges.
The single-sector scanner is a real time imaging system in which the elementary transducers of a linear array are excited in time sequence to generate angulated acoustic beams at many angles relative to the normal to the array at its midpoint. Echoes returning from targets in the direction of the transmitted beam arrive at the array elements at different times necessitating relative delaying of the received echo electrical signals by different amounts so that all the signals from a given point target are summed simultaneously by all elements of the array. In addition to beam steering delays, dynamic electronic focusing to improve image quality is achieved by additional channel-to-channel delay differences to compensate for propagation path time delay differences from a focal point to the array element positions. A cardiac scanner with dynamic time delay focusing to increment the range from which echoes are being received during a reception period is described by Thurstone and von Ramm in "A New Ultrasonic Imaging Technique Employing Two-Dimensional Electronic Beam Steering", Acoustical Holography, Vol. 5, 1974, Plenum Press, New York, pp. 249-259. In such prior art fixed aperture sector scanners, all the receive elements are active during every echo reception period and contribute to coherent summation to generate the focused echo signal. One problem with dynamic electronic focusing is that at short ranges less than the full aperture, dynamic focusing delays must be made so rapidly that it becomes impossible to keep up, and another problem is that such changes just before coherent summation have the potential of producing artifacts in the display.
A dynamic array aperture and electronic focus control for sector scanners and other imaging systems is disclosed and claimed in allowed copending commonly assigned application Ser. No. 864,597 filed on Dec. 27, 1977 by Charles E. Thomas. As the range from which echoes are being received propagates out, the array aperture during every echo reception period is increased by steps by switching in more transducer elements. At least one adjustment of focusing time delays is made to dynamically focus the echoes at different focal points. Improved lateral resolution is attained especially at ranges less than the full aperture by adjusting the aperture size incrementally.
The present invention is directed to a low cost single sector ultrasonic imager which does not require dynamic focusing. Conventional fixed focus, fixed aperture systems--with a physical lens achieved by curving the transducer array or by an acoustic lens in front of a flat array--focus the lens near the middle of the intended field of view. This produces degraded resolution at both the near and far extremes of the field of view, and to minimize this problem a relatively small aperture must be chosen. Consequently, the resolution is rather limited.